Wrongfully billed for an account!!!

i had to upgrade my account. i did the top up 500 account. At the MTN shop in potchefstroom mall.She/Monique did not activate it on the account. they cut off my service due to no payment. They admitted they made a mistake by making it an open account and according to them they \fixed"it. MTN SA said Potch mtn mall shop. must refund me. Since march 2015 my phone is cut off weekly due to no payments. although i load my own airtime and use my data like the top up 500 require. since then i made a total of R6304.58 payments for a topup account just to have my phone switched back on. an email was sent from MTN cape town informing us via email that Potch mtn must repay us.we did sent this mail to the and still weekly my phone is disconnected because i owe them money. They admitted it was a system failure therefor they will refund us. yet no one can or will assist us. and its a weekly problem now since march. please help us sort this out. i am an mtn customer for more then 15years. and this is the badest service i have ever experienced. its costing me money for paying the account and still need to load up airtime like normal top up. they say the credit should have been given to us already"""